How many Platy fish can I have?

I have a 50 gallon tank, cycled, two years old, good filters (2), heater. In it, I have - 1 Betta, 9 Cory Catfish, 2 Pleco's, 2 Algae Eaters, three Black Moors, and three Fancy Goldfish. I am considering re-homing the goldfish, and getting Platy fish. How many can I put in this tank, without the six goldfish, and not be over-crowding? Also, is it O.K. to mix the different 'types' of Platy fish?